The present invention concerns a process for the industrial synthesis of urea, making ammonia and carbon dioxide react in at least one reaction space, at high pressure and temperature, and recycling at least partially the unreacted products.
The invention also concerns the applications of this process to conventional systems in order to improve the urea yield and reduce energy consumption.
New or modified plants obtained by the implementation of the above-mentioned process and its applications to pre-existing systems are within the scope of the invention.
Several types of processes, systems and plants for the industrial synthesis of urea from NH.sub.3 (in excess) and CO.sub.2, with recycling after stripping, are known. Among the most prominent of these is the Snamprogetti process of isobaric stripping with NH.sub.3.
The main technical characteristics of the isobaric stripping process (Snamprogetti) can be summarized as follows:
synthesis pressure: approx. 150 bar PA0 NH.sub.3 /CO.sub.2 mol in the reactor: approx. 3.2-3.4 PA0 H.sub.2 O/CO.sub.2 mol in the reactor: approx. 0.6-0.7 PA0 temperature of the reactor: 190.degree. C. PA0 yield: approx. 62-63% PA0 steam compumption: approx. 900 kg/MT urea
Among the other processes with separate recycle of NH.sub.3 widely used in the past, those of Toyo Engineering Ltd., indicated with the abbreviation TRC - TRC-I (Total Recycle C, TRC Improved), can be mentioned, characterized by the fact that the ratio NH.sub.3 /CO.sub.2 in the reactor is equal to approx. 4-4.5 mol and the separation of NH.sub.3 at 18-20 bar is more impressive than the previous process (Snamprogetti), also for the lack of an isobaric stripping.
Another generation of processes has been developed recently for increasing the yield in the reactor of the so-called stripping processes (Snamprogetti NH.sub.3 selfstripping and Stamicarbon CO.sub.2 stripping), of the conventional values 55-65% up to 70-75%, providing at the same time the above-mentioned high-yield reactors with isobaric loops.
Reference is made to the new and more recent processes such as "IDR" (Isobaric Double Recycle) of Montedison and "ACES" of Toyo Engineering Ltd. The Snamprogetti and the above-referred to prior art processes are described in an article by Zardi in "Nitrogen", No. 135, January/February 1982, pp. 26-37.
In the above-mentioned recent processes "IDR" and "ACES", still operating with high ratios NH.sub.3 /CO.sub.2 in the high yield reactor, all the unaltered reactants, including the high excess of NH.sub.3, are recycled to the reactor under the form of carbamate solution. (The separate recycling of NH.sub.3 does not exist.)
Without doubt, much better yields are obtained (compared with the very first processes), but these advantages are coupled with a high complexity and complications in the construction of the equipment.
Furthermore, these processes have the inconvenience of having to strip the unreacted products under high temperature conditions with real risks of decomposition of the laboriously produced urea.
It is worth mentioning that in U.S. Pat. No. 4,670,588, a process with two reaction zones or reactors in series is described, the philosophy of which is, however, completely different from the one of the present invention.
Also in the "IDR" it is possible to utilize reactors with two isobaric zones in series functioning with diverse molar ratios of the reactants.
The first aim of the present invention is to provide a process without the above-mentioned inconveniences which can be operated easily, with total high yields and low consumptions.
Another aim of the invention is the applicability of this process to the conventional systems already having recycling sections for ammonia (or adapted with a new recycling section of the ammonia), reaching global yields of urea transformation in the synthesis sections equal or higher to the ones obtained with the modern processes "IDR", "ACES", etc.
A further aim of the invention is represented by the simple and effective plants (new or modified) resulting from the implementation of the process.